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Plastic Material
Typical Thermoset Molding Resins:In general, all thermosetting resins share certain characteristics and properties that distinguish them as a class: when heated to their cure point, they undergo a chemical change called cross-linking; once heated to this state, they solidify and hold their shape rigidly. Subsequent reheating will not melt the plastic or deform the shape. Thermoset resins were among the first organic plastics ever developed, and in the early days their dimensional stability and rigidity came at a price: they were also brittle – prone to cracking, chipping and crushing.Thankfully, today is a different story; modern compounds and filler materials have added impact resistance, structural strength, and scratch-resistant surface hardness to thermoset’s already impressive roster of properties. Even Plastic injection molding, a door once closed to thermoset materials, is now wide open – offering the same quick-cycle, low waste, high-volume processing that has become ubiquitous in the thermoplastics world.As a class, thermosetting materials have — to a greater or lesser degree, depending on the specific compound and filler additives used (if any) — several desirable properties:
Excellent Dielectrics (they’re electrical insulators)Good Sound Absorption (they’re sonically dead and acoustically neutral)Extreme Rigidity (courtesy of cross-linking in the curing process)Excellent Dimensional Stability (more cross-linking benefits)Good Structural Strength (especially when reinforced with fiber fillers)High Surface Hardness (with good scratch resistance, too)Decorative Surface Gloss (a phenolic knob from a polished mold can be a thing of beauty!)Resistance to Common Chemicals (although some strong oxidizers and reducing agents will attack them)Holds Molded-In Metal Inserts Securely (nothing short of smashing the plastic will release a properly designed molded-in metal insert)High Heat Tolerance (remember, once cross-linked and cured, thermoset materials won’t melt, making them well-suited for high temperature applications)Naturally Fire-Retardant, with Low Smoke (most thermoset plastics don’t readily burn)Different compounds of thermosetting resins make them suitable for multiple processing methods, and impart different properties to the finished product. Here are some of the basic groups of thermosetting materials we commonly plastic mold with here at Standard Plastic:
Resin heating
A constant supply of thermoplastic resin, usually in the form of powder, pellets or granules, is fed into the barrel of the injection unit through a large hopper. A secondary hopper introduces beads of colorant into the flow of resin. As the screw augers the plastic resin through the barrel from the hoppers to the nozzle, the resin is heated in two ways …
The resin is heated by shearing and compression. As the resin is augered through the barrel by the screw, shearing occurs as the resin is scraped from the inner walls of the barrel by the flights of the screw, and as the resin drags along the inner surfaces of both barrel and screw. Because of the tapered screw, the resin is also compressed as it is squeezed into the continually shrinking space between the barrel wall and the root of the screw. All this scraping, dragging and compression creates heat, which in turn melts the resin.
As the resin moves through the screw on its way from pellets to melt, it wraps around the screw in a long, thin ribbon of resin that keeps getting thinner (and more compressed, and hotter) as the screw root tapers through the feed and transition zones. As the screw flights scrape the melting resin from the inner barrel wall, they cause an internal resin flow inside the resin ribbon, which further contributes to shearing forces and heating.
The resin is also heated by the injection unit’s barrel, which is itself heated in three temperature “zones:”
- The FEED Zone — Not directly heated. In this zone, the plastic resin pellets are packing into the screw chamber, forming a long thin “ribbon” of resin material that wraps around the screw.
- The TRANSITION Zone — Heater bands around the barrel heat its middle part, which in turn starts heating the ribbon of resin. In this zone, also called the “mixing” zone or the “compression” zone, the resin begins to melt and changes from cold pellets into a warm slush.
- The METERING Zone — More heater bands bring the barrel and the resin fully up to melt temperature. In this zone, the resin becomes a hot, flowing fluid that is ready for injection into the plastic mould. This “melt” accumulates in the metering zone and the nozzle, where it is held at melt temperature until the reciprocating screw has metered the appropriate amount of melt for a full shot, at which point the screw rams the melt through the nozzle into the mold.
In thermoset injection molding the heated barrel is the primary heat source for melting the resin; heat from shearing and compression plays a negligible role.
In thermoplastics, the roles are reversed; shearing and compression make the major contribution to achieving melt temperature — the heated barrel serves more as a heat buffer, holding the plasticized resin above the melt point.
THERMOPLASTIC INJECTION MOLDING:
ADVANTAGES and DISADVANTAGES
In general, thermoset injection molding is good for high-volume production runs. If warehousing a high volume of parts is an issue for you (as it is with many of Standard Plastic’s clients), we can provide warehousing with delivery-on-demand from stock. Either way — all at once, or in scheduled JIT deliveries — high volume injection molding will typically give you a lower cost per part.
On the PLUS side …
- Lower part costs. The injection molding machines transform the raw resin directly into finished products in one continuous, automated cycle. Everything from refilling the hopper to metering the resin to opening and closing the mold and ejecting parts is fully mechanized by a team of machines. This all means: very low labor costs compared to other molding methods, which translates into lower part costs for you. (The one “labor cost” that we don’t skimp on is our skilled operators; the high quality and consistency that these injection molding machines offer depends on the knowledge and attention to detail that only well-trained human operators can provide.)
On the MINUS side …
- Higher mold cost, both in initial expense (thermoplastic injection molds are typically multi-cavity, and have a complex runner / gate system), and in upkeep.
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